Does it use the exact same gearbox as the Toyota Camry ? If so, (and I mean exact), TRD should have an item available in Japan. I've read they have one available for every FWD vehicle they've produced (Toyota Sceptre is the equivalent model in Japan). I doubt (unfortunately) that you'll find one in Australia, as it was never an option on the Camry or Apollo.
I'm not sure if the Celica GT4 uses some sort of LSD that you could use, but someone may be able to shed some more light on the subject for you.